System and method for time domain interpolation of signals for channel estimation

ABSTRACT

A system and method for time domain interpolation of signals for channel estimation. A method for computing channel estimates comprises storing symbols in a buffer, using time domain interpolation (TDI) for a first time to compute channel estimates for a set of sub-carriers of a symbol. The channel estimates are computed from the symbol and a first number of required symbols in the buffer. The method also comprises using TDI for a second time to compute channel estimates for the set of sub-carriers of a symbol. The channel estimates are computed from the symbol, a second number of required symbols in the buffer, and a buffered symbol used as a missing required symbol if the missing required symbol is not in the buffer.

TECHNICAL FIELD

The present invention relates generally to a system and method forwireless communications, and more particularly to a system and methodfor time domain interpolation of signals for channel estimation.

BACKGROUND

A wireless communications channel may cause arbitrary time dispersion,attenuation, and phase shift in a received signal. Channel estimationmay be used to form an estimate of the effects of the wirelesscommunications channel from available training data transmitted over thewireless communications channel. Since the training data is known, theeffects of the wireless communications channel on the training data maybe determined and used to remove the effects of the wirelesscommunications channel from the received signal.

FIG. 1 illustrates a prior art technique of using channel estimation toremove the effects of the wireless communications channel on a receivedsignal. A received signal at a receiver may be expressed as Y=Hx+n,where Y is the received signal, H is the wireless communicationschannel, x is the transmitted signal, and n is noise. A channelestimation unit 105 may process a received training signal, which isknown at the receiver, to produce a model of the wireless communicationschannel, Ĥ. The model of the wireless communications channel, Ĥ, may beprovided to a channel compensation unit 110, where it may be applied tothe received signal, Y, to produce a recovered transmitted signal,{circumflex over (x)}.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by embodiments of a systemand a method for time domain interpolation of signals for channelestimation.

In accordance with an embodiment, a method for computing channelestimates of a communications channel using time domain interpolation,wherein training data is transmitted over scattered sub-carriers, isprovided. The method includes storing symbols received over thecommunications channel in a buffer, and using time domain interpolationto compute channel estimates for a set of sub-carriers of a symbol beingprocessed for a first time. The sub-carriers are capable of containingtraining data for other symbols but do not contain training data for thesymbol being processed. The time domain interpolation computes thechannel estimates from the symbol being processed and a first number ofrequired symbols, in response to determining that all of the requiredsymbols are in the buffer. The method also includes using time domaininterpolation to compute channel estimates for the set of sub-carriersof the symbol being processed for a second time. The time domaininterpolation computes the channel estimates from the symbol beingprocessed, a second number of buffered required symbols, and a bufferedsymbol used as a missing required symbol, in response to determiningthat the missing required symbol is not in the buffer.

In accordance with another embodiment, a method for communicatingwirelessly is provided. The method includes buffering symbols receivedfrom a transmitter, computing channel estimates using the bufferedsymbols, recovering symbols from the buffered symbols using the channelestimates, and decoding data from the recovered symbols. The computingchannel estimates includes computing channel estimates for a set offirst sub-carriers in a symbol being processed with first sub-carrierscontaining training data, and using interpolation to compute channelestimates for sub-carriers in the symbol being processed not containingtraining data. The using interpolation includes using time domaininterpolation to compute channel estimates for a set of secondsub-carriers of the symbol being processed, and using frequency domaininterpolation to compute channel estimates for a set of thirdsub-carriers in the symbol. The second sub-carriers are capable ofcontaining training data for other symbols but do not contain trainingdata for the symbol being processed and the third sub-carriers do notcontain training data. The time domain interpolation uses the symbolbeing processed, buffered required symbols, and/or a buffered symbolused as a missing required symbol.

In accordance with another embodiment, a communications device isprovided. The communications device includes a transmitter fortransmitting data and training data over a number of sub-carriers, and areceiver for receiving transmitted data and training data. The receiverincludes a receive filter that attenuates interferers, a channelestimator coupled to the receive filter, and a baseband processorcoupled to the channel estimator. The channel estimator computes achannel estimate for a wireless communications channel using trainingdata contained in the received transmitted training data and thebaseband processor recovers the transmitted data from the receivedtransmitted data using the channel estimate and decodes and filters therecovered transmitted data. The channel estimator includes a time domaininterpolation unit coupled to a memory used to buffer received symbols,a frequency domain interpolation unit coupled to the memory, and abuffered symbol select unit coupled to the time domain interpolationunit. The time domain interpolation unit uses time domain interpolationto compute a channel estimate for sub-carriers of a symbol that arecapable of containing training data but not containing training data,the frequency domain interpolation unit computes a channel estimate forsub-carriers of the symbol not capable of containing training data, andthe buffered symbol select unit selects symbols from the memory, whereinthe symbols are temporally close to the symbol.

An advantage of an embodiment is that little additional hardware isrequired. This may result in an integrated solution with small chip areaand reduced power consumption.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the embodiments that follow may be better understood.Additional features and advantages of the embodiments will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of a prior art technique of using channel estimationused to remove the effects of a wireless communications channel;

FIG. 2 a is a diagram of a number of symbols in a WiMAX transmission,wherein the transmission utilizes a partially used sub-carrier (PUSC)permutation;

FIG. 2 b is a diagram of a number of symbols in a WiMAX transmission,wherein the transmission utilizes a fully used sub-carrier (FUSC)permutation;

FIG. 2 c is a diagram of a number of symbols in a WiMAX transmission,wherein the transmission utilizes an adaptive modulation and coding(AMC) permutation;

FIG. 3 is a diagram of a portion of a wireless communications device;

FIG. 4 is a diagram of a WiMAX frame;

FIG. 5 a is a diagram of a sequence of events in communicating over awireless communications channel;

FIG. 5 b is a diagram of a sequence of events in the time domaininterpolation of channel estimates;

FIG. 5 c is a diagram of a sequence of events in selecting a replacementsymbol;

FIG. 6 a is a diagram of several WiMAX symbols, highlighting thecomputing of channel estimates for a symbol;

FIG. 6 b is a diagram of several WiMAX symbols, highlighting thecomputing of channel estimates for a symbol;

FIG. 7 a is a diagram of several WiMAX symbols, highlighting time domaininterpolation;

FIG. 7 b is a diagram of several WiMAX symbols, highlighting frequencydomain interpolation;

FIG. 8 is a diagram of a sequence of WiMAX symbols near a zone switch;

FIGS. 9 a through 9 c are diagrams of sequences of WiMAX symbols near azone switch, highlighting different symbol combinations for computingchannel estimates using time domain interpolation;

FIGS. 10 a and 10 b are diagrams of WiMAX symbols transmitted in a WiMAXsystem using PUSC permutation and two transmit antennas; and

FIGS. 11 a and 11 b are diagrams of WiMAX symbols transmitted in a WiMAXsystem using AMC permutation and two transmit antennas.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments are discussed in detail below.It should be appreciated, however, that the present invention providesmany applicable inventive concepts that can be embodied in a widevariety of specific contexts. The specific embodiments discussed aremerely illustrative of specific ways to make and use the invention, anddo not limit the scope of the invention.

The embodiments will be described in a specific context, namely awireless receiver adherent to the Worldwide Interoperability forMicrowave Access (WiMAX) technical standard. The invention may also beapplied, however, to wireless communications protocols wherein trainingdata is transmitted along with transmitted data, such as the 3^(rd)Generation Partnership Project (3GPP) Long Term Evolution (LTE) wirelesscommunications protocol.

Channel estimation may occur several different ways. Training data maybe transmitted by a transmitter to a receiver before or aftertransmitted data is transmitted to the receiver. This may allow for anestimation of the entire wireless communications channel withoutinterfering with transmitted data. Alternatively, the training data maybe transmitted along with the transmitted data. For example, in a WiMAXcompliant wireless communications system, training data may betransmitted in scattered sub-carriers (pilots), interleaved with thetransmitted data. To improve the channel estimation, the location of thescattered pilots may change from one symbol to the next. The scatteredpilots may also be scattered in different patterns depending onpermutation.

With reference now to FIG. 2 a, there is shown a diagram illustrating anumber of symbols in a WiMAX transmission, wherein the transmissionutilizes a partially used sub-carrier (PUSC) permutation. The symbols ina WiMAX transmission may utilize orthogonal frequency divisionmultiplexing (OFDM). The diagram illustrates the content of sub-carriersfor a number of symbols, including a first odd symbol 205, a first evensymbol 206, a second odd symbol 207, and a second even symbol 208. Inthe PUSC permutation, each cluster comprises 14 sub-carriers in thefrequency domain and spans two symbols in the time domain, for example,cluster 209 includes the first odd symbol 205 and the first even symbol206 in the time domain and spans 14 sub-carriers in the frequencydomain. Each sub-carrier within the cluster, such as sub-carrier 210,may contain transmitted data (shown in FIG. 2 a as hollow circles),while some sub-carriers, such as sub-carrier 211, may also containtraining data (shown in FIG. 2 a as cross-hatched circles). Asub-carrier containing training data may be referred to as a pilot. Apilot pattern may be repeated every other OFDM symbol. A sub-carriercapable of containing training data may contain training data in onesymbol may not contain training data in another symbol. For example,sub-carrier 211 may contain training data in the first odd symbol 205but may not contain training data in the first even symbol 206.

In the PUSC permutation, a sub-carrier capable of containing trainingdata may contain training data in every other symbol. For example, thesub-carrier 211 may contain training data in the first odd symbol 205and the second odd symbol 207, but in the first even symbol 206 and thesecond even symbol 208, it contains transmitted data. Similarly, asub-carrier 212 may contain transmitted data in the first odd symbol 205and the second odd symbol 207, but in the first even symbol 206 and thesecond even symbol 208, it contains training data.

FIG. 2 b illustrates a number of symbols in a WiMAX transmission,wherein the transmission utilizes a fully used sub-carrier (FUSC)permutation. The diagram illustrates the content of sub-carriers for anumber of symbols, including a first odd symbol 210, a first even symbol211, a second odd symbol 212, and a second even symbol 213. In the FUSCpermutation, the pilot repeats every 12-th sub-carrier. Moreover, thereis a six sub-carrier offset in the pilot location between odd and evensymbols. Each sub-carrier, such as sub-carrier 215, may containtransmitted data, while some sub-carriers, such as sub-carrier 216 andsub-carrier 217, may also contain training data. In the FUSCpermutation, a sub-carrier capable of containing training data maycontain training data in every other symbol. For example, thesub-carrier 216 may contain training data in the first even symbol 211and the second even symbol 213, but in the first odd symbol 210 and thesecond odd symbol 212, it contains transmitted data. Similarly, thesub-carrier 217 may contain training data in the first odd symbol 210and the second odd symbol 212, but in the first even symbol 211 and thesecond even symbol 213, it contains transmitted data. A pilot patternmay be repeated every other OFDM symbol.

FIG. 2 c illustrates a number of symbols in a WiMAX transmission,wherein the transmission utilizes an adaptive modulation and coding(AMC) permutation. The diagram illustrates the content of sub-carriersfor a number of symbols, including a first odd symbol 220, a first evensymbol 221, a second odd symbol 222, a second even symbol 223, and athird odd symbol 224. In the AMC permutation, the pilot repeats every9-th sub-carrier. Moreover, there is a three sub-carrier offset in thepilot location between two consecutive symbols in time, i.e., a pilotpattern may be repeated every third OFDM symbol. Each sub-carrier, suchas sub-carrier 225, may contain transmitted data, while somesub-carriers, such as sub-carrier 226, sub-carrier 227, and sub-carrier228, may also contain training data. In the AMC permutation, asub-carrier capable of containing training data may contain trainingdata in every third symbol. For example, the sub-carrier 226 may containtraining data in the first odd symbol 220 and the second even symbol223, but in the first even symbol 221 and the second odd symbol 222, itcontains transmitted data. Similarly, the sub-carrier 217 may containtraining data in the second odd symbol 222, but in the first odd symbol220, the first even symbol 221, the second even symbol 223, and thethird odd symbol 224, it contains transmitted data. The sub-carrier 228may contain training data in the first even symbol 221 and the third oddsymbol 224, but in the first odd symbol 220, the second odd symbol 222,and the second even symbol 223, it contains transmitted data.

FIG. 3 illustrates a view of a portion of a wireless communicationsdevice 300. The wireless communications device 300 includes atransmitter 305 and a receiver 310. The transmitter 305 and the receiver310 may share an antenna 315 or they each may have dedicated antennas.Although shown as a single antenna, the antenna 315 may comprisemultiple antennas with the transmitter 305 and/or the receiver 310 usingmore than one antenna to transmit and/or receive multiple independentdata streams. The wireless communications device 300 with thetransmitter 305 and the receiver 310 using multiple antennas to transmitand/or receive multiple independent data streams may be referred to as amultiple input, multiple output (MIMO) wireless communications device300.

The receiver 310 includes a receive filter 320 that may be used tofilter the received signal and potentially eliminate (or attenuate)interferers and/or noise received along with the received signal. Thereceiver 310 also includes a channel estimator 325 to compute a channelestimate for the wireless communications channel and to use the channelestimate to recover the transmitted signal from the received signal. Amemory 330 coupled to the channel estimator 325 may be used to store thechannel estimate, received symbols, and so forth. A baseband processor335 may be used to use the channel estimate to recover the transmittedsignal from the received signal, perform signal decoding, errordetection and correction, additional filtering, amplification, and otherforms of signal processing on the received signal to produce data foruse by the receiver 310 or other electronic devices attached to thereceiver 310.

The channel estimator 325 includes a time domain interpolation (TDI)unit 340 for use in performing interpolation of the channel estimates ofa sub-carrier between symbols in the time domain, a frequency domaininterpolation (FDI) unit 345 for use in performing interpolation of thechannel estimates between sub-carriers of a symbol in the frequencydomain, and a buffered symbol select unit 350. The buffered symbolselect unit 350 may be used to select received symbols that may bebuffered in the memory 330 for use in TDI by the TDI unit 340. Thebuffered symbol select unit 350 may be used to select the symbols to beused in TDI based on the permutation used in the received transmission,and if needed symbols are not available, the buffered symbol select unit350 may select buffered symbols to duplicate (replicate) to replace theunavailable symbols. A more detailed description of the operation of thebuffered symbol select unit 350 is provided below.

If multiple antennas are used to transmit and receive multipleindependent data streams, then the number of symbols that may need to bebuffered in the memory 330 may need to be increased. However, thetraining data transmitted in sub-carriers (pilots) may be orthogonallymultiplexed so that the training data may be applied to a single antennaat a time (i.e., a sub-carrier assigned as a pilot for one antenna maybe transmitting null data for other antennas). This may increase thenumber of symbols at a receiver over which the pilots are repeated,thereby potentially increasing the symbol buffering requirement.

FIG. 4 is a diagram of a WiMAX frame 400. A WiMAX frame may span acrossmultiple permutation zones, with a zone being a sequence of symbolsusing a single permutation. The WiMAX frame 400 includes a preamble 405and a first zone 410 that by definition must use the PUSC permutation.When there is a permutation change, there is a zone switch “ZN SW,” suchas a first zone switch 412 between the preamble 405 and the first zone410. If there are no additional permutation changes, then a WiMAX framewill have only a single zone.

However, if there are subsequent permutation changes, then a WiMAX framemay have addition zones. Additional zones in a WiMAX frame may use thePUSC, the FUSC, or the AMC permutations. For example, a second zone 415in the WiMAX frame 400 may use the PUSC, the FUSC, or the AMCpermutations. However, if a zone uses the same permutation as animmediately preceding zone, then the two zones may be considered thesame zone. A second zone switch 417 occurs between the first zone 410and the second zone 415.

FIG. 5 a illustrates a sequence of events 500 in communicating over awireless communications channel, wherein the communications includestraining data that is scattered over a range of pilots. Thecommunications may occur continuously during normal operation of awireless communications device since the training data is interleavedwith transmitted data and scattered over a range of pilots.

The communications may begin with a buffering of received symbols (block505). The number of received symbols to be buffered may depend uponseveral factors, including an allowable amount of latency, availablememory for buffering, the permutation used in the transmissions, and soforth. For example, for PUSC and FUSC permutations wherein pilots arerepeated every other symbol, a minimum number of symbols buffered may bethree. However, for AMC permutations wherein pilots are repeated everythird symbol, a minimum number of symbols buffered may be five.

As symbols are received, a channel estimate may be computed for thechannel at sub-carriers corresponding to the scattered pilots containingtraining data (block 510). FIG. 6 a is a diagram of a number of symbolsin a WiMAX transmission, wherein the transmission utilizes the PUSCpermutation. FIG. 6 a highlights the computing of the channel estimatefor symbol 600. Since the transmission uses the PUSC permutation, threesymbols may be needed to compute channel estimates. Since the pilotscontain actual training data, no interpolation, either time domain orfrequency domain, is required. The computing of the channel estimate atthe pilots may be expressed mathematically as:

${{\hat{H}}_{k,l} = \frac{Y_{k,l}}{X_{k,l}}},$where Y_(k,l) is the received data at pilot location k,l and X_(k,l) isthe known training data for the pilot location k,l and k and l areinteger values representing symbol and sub-carrier location,respectively. For example, in highlight 605 and highlight 610, for thesymbol 600, sub-carriers 606 and 611 are pilots, therefore, channelestimates for the sub-carriers may be directly computed. However, inhighlight 615 and highlight 620, sub-carriers 616 and 621 are notpilots, therefore, channel estimates for the sub-carriers may not bedirectly computed. Similarly, FIG. 6 b is a diagram of a number ofsymbols in a WiMAX transmission, wherein the transmission uses the AMCpermutation. Channel estimates for sub-carriers 650 and 655 may becomputed directly since sub-carriers 650 and 655 are pilots, however,since sub-carriers 660, 665, 670, and 675 are not pilots, the channelestimates may not be directly computed.

Turning back to FIG. 5 a, after computing the channel estimate forsub-carriers at the scattered pilots, time domain interpolation may beperformed using buffered symbols to compute channel estimates ofsub-carriers carrying both training data and transmitted data (block515). Time domain interpolation may involve the computation of channelestimates for sub-carriers that are used to carry transmitted data butmay also be used to carry training data (i.e., the sub-carriers that inone symbol may carry transmitted data but in another symbol may be apilot). Time domain interpolation uses sub-carriers and pilots fromdifferent symbols. FIG. 7 a is a diagram of a number of symbols in aWiMAX transmission, wherein the transmission utilizes the PUSCpermutation. For example, time domain interpolation may be used tocompute a channel estimate for sub-carrier 705, which in a precedingsymbol was a pilot (pilot 710) and in a succeeding symbol will be apilot (pilot 715). The computing of the channel estimate for sub-carrier705 may use channel estimates from the pilot 710 and the pilot 715.Similarly, for the following symbol, time domain interpolation may beused to compute a channel estimate for sub-carrier 720, which in apreceding symbol was a pilot (pilot 721) and in a succeeding symbol willbe a pilot (pilot 722). Channel estimates of sub-carriers computed usingtime domain interpolation are shown in FIG. 7 a as circles hatched withvertical lines, such as sub-carrier 705 and sub-carrier 720.

Turning back to FIG. 5 a, after performing the time domaininterpolation, frequency domain interpolation may be performed tocompute channel estimates of sub-carriers carrying only transmitted data(block 520). Frequency domain interpolation may involve the computationof channel estimates for sub-carriers of a symbol that are used to carrytransmitted data using channel estimates from pilots and the channelestimates from the time domain interpolation. FIG. 7 b is a diagram of anumber of symbols in a WiMAX transmission, wherein the transmissionutilizes the PUSC permutation. For example, frequency domaininterpolation may be used to compute channel estimates for sub-carriers755, 756, and 757 (as well as other sub-carriers in the symbol notalready having a channel estimate) using channel estimates fromsub-carriers 760 and 761. The channel estimate for sub-carrier 760 wasobtained previously using time domain interpolation, while sub-carrier761 is a pilot. Similarly, frequency domain interpolation may be used tocompute channel estimates for sub-carriers 765, 766, and 767 (as well asother sub-carriers in the symbol not already having a channel estimate)using channel estimates from sub-carriers 770 and 771. The channelestimates for sub-carriers 770 and 771 were obtained previously usingtime domain interpolation. Channel estimates for sub-carriers computedusing frequency domain interpolation are shown in FIG. 7 b as circleshatched with horizontal lines, such as sub-carrier 755 and sub-carrier765.

Turning back now to FIG. 5 a, after the time domain interpolation andthe frequency domain interpolation completes (blocks 515 and 520), thechannel estimation may be used to recover the received symbols and thereceived symbols may be decoded to recover transmitted data (block 525).The communications may continue as long as symbols continue to bereceived.

FIG. 5 b illustrates a sequence of events 550 in the time domaininterpolation of channel estimates, wherein training data is scatteredover a range of pilots. As discussed previously, time domaininterpolation may require the buffering of several symbols, with thenumber of symbols required varying depending on the permutation. Forexample, in a simple configuration, for PUSC and FUSC, a requiredminimum number of symbols to be buffered is three: the symbol that timedomain interpolation is computing channel estimates for (i.e., thesymbol being processed), a symbol immediately preceding the symbol beingprocessed, and a symbol immediately succeeding the symbol beingprocessed. For AMC, a required minimum number of symbols to be bufferedis five: the symbol that time domain interpolation is computing channelestimates for, two symbols immediately preceding the symbol beingprocessed, and two symbols immediately succeeding the symbol beingprocessed. For other permutations and other configurations of pilots, adifferent number of symbols may need to be buffered. Furthermore, timedomain interpolation may utilize more than a required minimum number ofsymbols to achieve channel estimates of potentially higher quality, forexample.

Since time domain interpolation computes channel estimates usingmultiple symbols, problems may arise at a zone switch, where the timedomain interpolation of a symbol may not correctly produce a desiredresult since available symbols may have different permutations. FIG. 8illustrates a view of symbols in a WiMAX frame near or at a zone switch,wherein symbols S(K−1) 705, S(K−2) 706, S(K−3) 707, S(K−4) 708, andS(K−5) 709 are symbols belonging to a first zone “zone one,” whilesymbols S(K) 715, S(K+1) 716, S(K+2) 717, S(K+3) 718, and S(K+4) 719belong to a second zone “zone two.” Since symbols S(K−1) 705 to S(K−5)709 and symbols S(K) to S(K+4) belong to different zones, they havedifferent permutations. Therefore, if time domain interpolation attemptsto compute channel estimates of symbols near or at the zone switch, thenthe time domain interpolation may be incorrect since pilots of symbolshaving different permutations may not be at expected locations, ordifferent training data may be utilized.

For example, time domain interpolation of a channel estimate forsub-carriers in the symbol S(K−1) 705, wherein the first zone used FUSCpermutation may require the use of three symbols: the symbol S(K−1) 705;a symbol immediately preceding the symbol S(K−1) 705, S(K−2) 706; and asymbol immediately succeeding the symbol S(K−1) 705, S(K) 715. However,the symbol S(K) 715 belongs to the second zone and uses a differentpermutation. Therefore, the use of the symbol S(K) 715 may result in aninaccurate channel estimate.

Turning back to FIG. 5 b, after buffering all required symbols, the timedomain interpolation of channel estimates for the symbol being processedmay begin with a check to determine if all buffered symbols have thesame permutation (block 555). If all of the buffered symbols have thesame permutation, then time domain interpolation may be used to computechannel estimates for the symbol being processed (block 560). Zoneinformation is available in the DL-MAP, which is transmitted followingthe preamble in the WiMAX frame (as shown in FIG. 4).

However, if not all of the buffered symbols have the same permutation(block 555), then some of the buffered symbols may not be usable and itmay be necessary to replace some of the buffered symbols with areplacement symbol(s) (block 565). The replacement symbol(s) used toreplace some of the buffered symbols may have the following desiredcharacteristics: the replacement symbol(s) may be temporally close tothe symbol being processed to help minimize the effect of changes in thecommunications channel, the replacement symbol(s) may have the samepermutation as the symbol being processed, if more than one replacementsymbols are used, then a different symbol may be used for eachreplacement symbol, the replacement symbol may have the same pilotarrangement as the symbol being replaced, and so forth. A detaileddiscussion of the replacement of buffered symbols with a replacementsymbol(s) is provided below. Once the buffered symbols have beenreplaced, the time domain interpolation of channel estimates for thesymbol being processed may be computed (block 570).

FIG. 5 c illustrates a sequence of events 580 for use in selecting areplacement symbol. The selection of the replacement symbol may need tomeet the above listed criteria in order to ensure that the channelestimate(s) computed using time domain interpolation provides a usableresult. The selecting of the replacement symbol may begin with adetermination of a symbol that requires replacement (block 585). Ingeneral, a symbol may require replacement if (1) it is needed incomputing the time-domain interpolated channel estimates of the symbolbeing processed and (2) it has a different permutation from the symbolbeing processed. After determining the symbol requiring replacement, abuffered symbol having the same permutation as well as pilot locationsas the symbol requiring replacement (and also the symbol beingprocessed) that is closest (temporally) to the symbol being processedmay be selected as the replacement symbol (block 590). The replacementsymbol may be temporally close to the symbol being processed to helpminimize time dependent behavior that may be present in the wirelesscommunications channel. If there are any additional symbols requiringreplacement, the selecting of a replacement symbol may be repeated(block 595). The selecting of replacement symbols for situations whereinthere are multiple symbols requiring replacement may take place onesymbol at a time or in parallel.

As discussed previously, the use of time domain interpolation to computechannel estimates for the symbol being processed may require thereplacement of a buffered symbol(s) with a replacement symbol(s) whenthe symbol being processed is near or at a zone switch. The selection ofreplacement symbols, the number of replacement symbols used, and soforth may be based on the permutation and location of the symbol beingprocessed.

FIG. 9 a illustrates a view of symbols in a WiMAX frame near or at azone switch, wherein symbols in a first zone (symbols S(K−1) 705 toS(K−5) 709) use either the PUSC or FUSC permutation and symbols in asecond zone (symbols S(K) 715 to S(K+4) 719) use either the FUSC or PUSCpermutation. Since PUSC or FUSC is being used, three symbols need to bebuffered for the computing of a channel estimate using time domaininterpolation. When all three symbols share a common permutation, suchas the symbol S(K−4) 708, S(K−5) 709, and S(K−3) 707, shown as span 905,the use of time domain interpolation to compute the channel estimate forthe symbol S(K−4) 708 may be expressed as:h(k−4)=f(h(k−5),h(k−4),h(k−3)),where h(k−4) is the channel estimate for symbol S(K−4), and f( ) is thetime domain interpolation.

However, for a symbol at a boundary of a zone, such as the symbol S(K−1)705, the computation of the channel estimate using time domaininterpolation may require the symbols S(K−2) 706 and S(K) 715.Unfortunately, the symbol S(K) 715 is in a different zone and thereforehas a different permutation. A replacement symbol may be needed toreplace the symbol S(K) 715 in the computation of the channel estimatefor the symbol S(K−1) 705.

As discussed previously, the replacement symbol may be temporally closeto the symbol being processed to help minimize the effect of changes inthe communications channel, the replacement symbol(s) may have the samepermutation as the symbol being processed, if more than one replacementsymbols are used, then a different symbol may be used for eachreplacement symbol, the replacement symbol may have the same pilotarrangement as the symbol being replaced, and so forth. Using thesecriteria, the symbol S(K−2) 706 may be used as the replacement symbolfor the symbol S(K) 715 (shown as span 920). The use of time domaininterpolation to compute the channel estimate for the symbol S(K−1) 705(shown as span 910) may be expressed as:h(k−1)=f(h(k−2),h(k−1),h(k−2)).

Similarly, when all three symbols in the second zone share a commonpermutation, such as the symbol S(K+2) 717, S(K+3) 718, and S(K+4) 719,shown as span 915, the use of time domain interpolation to compute thechannel estimate for the symbol S(K+3) may be expressed as:h(k+3)=f(h(k+2),h(k+3),h(k+4)),while the use of time domain interpolation to compute the channelestimate for the symbol S(K) may require the use of a replacement symbolfor the symbol S(K−1) 705, which belongs in the first zone, may beexpressed as:h(k)=f(h+1),h(k),h(k+1)).

FIG. 9 b illustrates a view of symbols in a WiMAX frame near or at azone switch, wherein symbols in a first zone (symbols S(K−1) 705 toS(K−5) 709) use either the PUSC or FUSC permutation and symbols in asecond zone (symbols S(K) 715 to S(K+4) 719) use the AMC permutation.The computation of channel estimates for symbols in the first zone(symbols S(K−1) 705 to S(K−5) 709) may proceed as described in FIG. 9 a.

When AMC is being used, five symbols need to be buffered for thecomputing of a channel estimate using time domain interpolation. Whenall five symbols share a common permutation, such as symbol S(K) 715,S(K+1) 716, S(K+2) 717, S(K+3) 718, and S(K+4) 719, the use of timedomain interpolation to compute the channel estimate for the symbolS(K+2) 717 may be expressed as:h(k+2)=f(h(k),h(k+1),h(k+2),h(k+3),h(k+4)).

However, for symbols at a boundary of a zone, such as the symbol S(K)715 and the symbol S(K+1) 716, the computation of the channel estimateusing time domain interpolation may require the use of replacementsymbols since at least one of the symbols required in time domaininterpolation belongs in the first zone. For the computation of thechannel estimate of the symbol S(K) 715, two replacement symbols areneeded to replace symbols S(K−1) 705 and S(K−2) 706, and for thecomputation of the channel estimate of the symbol S(K+1) 716, onereplacement symbol is needed to replace symbol S(K−1) 705. Using thepreviously discussed replacement symbol criteria for the symbol S(K)715, the replacement symbols may be the symbol S(K+2) 717 to replace thesymbol S(K−1) 705 and the symbol S(K+1) 716 to replace the symbol S(K−2)706, while for the symbol S(K+1) 716, the replacement symbol may be thesymbol S(K+2) 717 to replace the symbol S(K−1) 705. The use of timedomain interpolation to compute the channel estimate for the symbolsS(K) 715 (shown as span 920) and the symbol S(K+1) 716 (shown as span925) may be expressed as:h(k)=f(h(k+1),h(k+2),h(k),h(k+1),h(k+2)) andh(k+1)=f(h(k+2),h(k),h(k+1),h(k+2),h(k+3)).

FIG. 9 c illustrates a view of symbols in a WiMAX frame near or at azone switch, wherein symbols in a first zone (symbols S(K−1) 705 toS(K−5) 709) use the AMC permutation and symbols in a second zone(symbols S(K) 715 to S(K+4) 719) use either the PUSC or FUSCpermutation. The computation of channel estimates for symbols in thefirst zone (symbols S(K−1) 705 to S(K−5) 709) may proceed as describedin FIG. 9 b. With the computation of channel estimate for the symbolS(K−1) 705, shown as span 930, being similar to the computation of thechannel estimate for the symbol S(K) 715 (span 920) shown in FIG. 9 b,and the computation of the channel estimate for the symbol S(K−2) 706,shown as span 935, being similar to the computation of the channelestimate for the symbol S(K+1) 925 shown in FIG. 9 b. The computation ofchannel estimates for symbols in the second zone (symbols S(K) 715 toS(K+4) 719) may proceed as described in FIG. 9 a.

FIGS. 10 a and 10 b illustrate views of symbols in a WiMAX transmission,wherein the WiMAX transmission is from a 2×2 MIMO system utilizing thePUSC permutation. In a 2×2 MIMO system, two transmit antennas may beused to transmit two independent data streams. FIG. 10 a illustrates aview of symbols from a first transmit antenna and FIG. 10 b illustratesa view of symbols from a second transmit antenna. Since there are twoindependent data streams, clusters, such as cluster 1005, may containfour symbols (two symbols from each independent data stream). As shownin FIG. 10 a, pilots, such as pilot 1010 and pilot 1015, may be used ata receive antenna to estimate the communications channel from the firsttransmit antenna to the receiver, while pilots, such as pilot 1020 andpilot 1025, may not be used and null data may be transmitted in theirplace. Similarly, as shown in FIG. 10 b, pilots, such as pilot 1030 andpilot 1035, may be used at a receive antenna to estimate thecommunications channel from the second transmit antenna to the receiver,while pilots, such as pilot 1040 and pilot 1045, may not be used andnull data may be transmitted in their place.

FIGS. 11 a and 11 b illustrate views of symbols in a WiMAX transmission,wherein the WiMAX transmission is from a 2×2 MIMO system utilizing theAMC permutation. FIG. 11 a illustrates a view of symbols from a firsttransmit antenna and FIG. 11 b illustrates a view of symbols from asecond transmit antenna. As shown in FIG. 11 a, pilots, such as pilot1105 and pilot 1110, may be used at a receive antenna to estimate thecommunications channel from the first transmit antenna to the receiver,while pilots, such as pilot 1115 and pilot 1120, may not be used andnull data may be transmitted. Similarly, as shown in FIG. 11 b, pilots,such as pilot 1125 and pilot 1130, may be used at a receive antenna toestimate the communications channel from the second transmit antenna tothe receiver, while pilots, such as pilot 1135 and pilot 1140, may notbe used and null data may be transmitted.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

1. A method for computing channel estimates of a communications channelusing time domain interpolation, wherein training data is transmittedover scattered sub-carriers, the method comprising: storing symbolsreceived over the communications channel in a buffer; using time domaininterpolation a first time to compute channel estimates for a set ofsub-carriers of a symbol being processed, wherein the sub-carriers arecapable of containing training data for other symbols but do not containtraining data for the symbol being processed, wherein the time domaininterpolation computes the channel estimates from the symbol beingprocessed and a first number of required symbols, in response todetermining that all of the required symbols are in the buffer; andusing time domain interpolation a second time to compute channelestimates for the set of sub-carriers of the symbol being processed,wherein the time domain interpolation computes the channel estimatesfrom the symbol being processed, a second number of buffered requiredsymbols, and a buffered symbol used as a missing required symbol, inresponse to a determining that the missing required symbol is not in thebuffer.
 2. The method of claim 1, further comprising, before using timedomain interpolation for the second time, selecting the buffered symbolfrom the buffer to replace the missing required symbol.
 3. The method ofclaim 2, wherein selecting the buffered symbol from the buffer compriseschoosing a symbol from the buffer having substantially the samesub-carrier and training data configuration as the missing requiredsymbol.
 4. The method of claim 3, wherein selecting the buffered symbolfrom the buffer further comprises choosing a symbol from the bufferclosest in time to the symbol being processed in response to determiningthat more than one symbol from the buffer has substantially the samesub-carrier and training data configuration as the missing requiredsymbol.
 5. The method of claim 2, wherein there are multiple missingrequired symbols, and wherein selecting the buffered symbol compriseschoosing a different buffered symbol for each missing required symbol.6. The method of claim 1, wherein the first number of required symbolsis based on a sub-carrier and training data configuration used in thesymbols received over the communications channel.
 7. The method of claim1, wherein the symbol being processed is also stored in the buffer. 8.The method of claim 1, wherein using time domain interpolation for thefirst time comprises using training data from the set of sub-carriers inthe required symbols to compute channel estimates for sub-carriers inthe set of sub-carriers in the symbol being processed.
 9. The method ofclaim 1, wherein using time domain interpolation for the second timecomprises using training data from the set of sub-carriers in therequired symbols and the buffered symbol used as the missing requiredsymbol to compute channel estimates for sub-carriers in the set ofsub-carriers in the symbol being processed.
 10. The method of claim 1further comprising after using time domain interpolation for the secondtime: selecting a new symbol being processed; and repeating the storing,using time domain interpolation for the first time, and using timedomain interpolation for the second time for the new symbol beingprocessed.
 11. The method of claim 1, wherein the first number ofrequired symbols or the second number of buffered symbols plus themissing required symbol is greater than a minimum required number ofsymbols needed in time domain interpolation.
 12. A method forcommunicating wirelessly, the method comprising: buffering symbolsreceived from a transmitter; computing channel estimates using thebuffered symbols, wherein computing channel estimates comprises:computing channel estimates for a set of first sub-carriers in a symbolbeing processed, wherein first sub-carriers contain training data, andusing interpolation to compute channel estimates for sub-carriers in thesymbol being processed not containing training data, wherein usinginterpolation comprises: using time domain interpolation to computechannel estimates for a set of second sub-carriers of the symbol beingprocessed, wherein the second sub-carriers are capable of containingtraining data for other symbols but do not contain training data for thesymbol being processed, wherein the time domain interpolation uses thesymbol being processed, buffered required symbols, and using frequencydomain interpolation to compute channel estimates for a set of thirdsub-carriers in the symbol, wherein the third sub-carriers do notcontain training data; recovering symbols from the buffered symbolsusing the channel estimates; and decoding data from the recoveredsymbols.
 13. The method of claim 12, wherein using time domaininterpolation comprises: using time domain interpolation to computechannel estimates for the set of second sub-carriers of the symbol beingprocessed, wherein the time domain interpolation uses the symbol beingprocessed and a number of buffered symbols temporally adjacent to thesymbol being processed in response to a determining that the symbolbeing processed and the number of buffered symbols have the samesub-carrier and training data configuration; and using time domaininterpolation to compute channel estimates for the set of secondsub-carriers of the symbol being processed, wherein the time domaininterpolation uses the symbol being processed, a number of bufferedsymbols temporally adjacent to the symbol being processed, and abuffered symbol in response to a determining the symbol being processed,the number of buffered symbols, and the buffered symbol have the samesub-carrier and training data configuration and that the number ofbuffered symbols is less than a number of symbols required to performtime domain interpolation.
 14. The method of claim 13, wherein the timedomain interpolation additionally uses a buffered symbol as a missingrequired symbol, wherein a missing symbol is a symbol required for timedomain interpolation but which has not been buffered, wherein thebuffered symbol is selected from the buffered symbols received from thetransmitter, and wherein the buffered symbol is closest temporally tothe symbol being processed having the same sub-carrier and training dataconfiguration as the missing symbol.
 15. The method of claim 14, whereinthere is more than one missing symbol, and wherein a different bufferedsymbol is selected for each missing symbol.
 16. The method of claim 12,wherein computing channel estimates for the set of first sub-carriersmay be expressed as: ${{\hat{H}}_{k,l} = \frac{Y_{k,l}}{X_{k,l}}},$where Ĥ_(k,l) is the channel estimate for a first sub-carrier locationk,l and Y_(k,l) is the training data at sub-carrier location k,l andX_(k,l) is the known training data for the pilot location k,l and k andl are integer values.
 17. A communications device comprising: atransmitter for transmitting data and training data over a number ofsub-carriers; a receiver for receiving transmitted data and trainingdata, the receiver comprising: a receive filter configured to attenuateinterferers; a channel estimator coupled to the receive filter, thechannel estimator configured to compute a channel estimate for awireless communications channel using training data contained in thereceived transmitted training data, the channel estimator comprising: atime domain interpolation unit coupled to a memory used to bufferreceived symbols, the time domain interpolation unit configured to usetime domain interpolation to compute a channel estimate for sub-carriersof a symbol that are capable of containing training data but notcontaining training data; a frequency domain interpolation unit coupledto the memory, the frequency domain interpolation unit configured tocompute a channel estimate for sub-carriers of the symbol not capable ofcontaining training data; a buffered symbol select unit coupled to thetime domain interpolation unit, the buffered symbol select unitconfigured to select symbols from the memory, wherein the symbols aretemporally close to the symbol; and a baseband processor coupled to thechannel estimator, the baseband processor configured to recover thetransmitted data from the received transmitted data using the channelestimates from both the time domain interpolation and frequency domaininterpolation, the baseband processor also configured to decode therecovered transmitted data.
 18. The communications device of claim 17,wherein the buffered symbol select unit is configured to select bufferedsymbols having the same training data and sub-carrier configuration asthe symbol.
 19. The communications device of claim 18, wherein thebuffered symbol select unit is further configured to select symbolsbuffered in the memory to replace symbols that are temporally close tothe symbol but are not in the memory.
 20. The communications device ofclaim 19, wherein the selected symbols buffered in the memory have thesame training data and sub-carrier configuration as the symbols that arenot in the memory.
 21. The communications device of claim 17, whereinthe communications device includes multiple transmit antenna andmultiple receive antennas, and wherein the training data areorthogonally multiplexed.